Shape-Selective Synthesis and Optical Properties of Highly Ordered One-Dimensional ZnS Nanostructures Liang Shi,*,† Yeming Xu,‡ and Quan Li‡ Department of Chemistry, UniVersity of Science and Technology of China, Hefei 230026, P. R. China, and Department of Physics, The Chinese UniVersity of Hong Kong, Shatin, New Territory, Hong Kong, P. R. China
CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 5 2214–2219
ReceiVed August 23, 2008; ReVised Manuscript ReceiVed January 22, 2009
ABSTRACT: Controlled fabrication of large-scale ZnS arrays of well-aligned 1D nanostructures is reported, which is carried out via a convenient one-step, wet-chemical approach without using a surfactant or template. Highly oriented ZnS nanowire, nanotube, or nanoribbon arrays can be selectively grown on Zn foil by simply modulating reaction temperature and sulfur concentration. The as-prepared ZnS 1D nanostructures are all single crystalline and have a uniform [0001] preferential growth direction. The growth mechanisms of 1D ZnS nanostructure arrays with different shapes are also discussed on the basis of the thermodynamics and kinetics control of diffusion, nucleation, and growth. Room-temperature luminescence properties of these well-ordered ZnS 1D nanostructure arrays are also studied. Introduction As an important II-VI group semiconductor, zinc sulfide (ZnS) has attracted a great deal of interest during the past decades due to its various unique properties. With a wide band gap of ∼3.7 eV at 300 K, ZnS is now used extensively in electroluminescent devices, flat panel displays, infrared windows, sensors, and lasers.1-5 In addition, ZnS has been employed effectively to explore the intrinsic recombination processes in dense excitonic systems because of its large exciton binding energy (40 meV) and small Bohr radius (2.4 nm). A number of luminescent properties have been realized by doping ZnS with appropriate ions, which makes it an excellent phosphor host material.6-9 Recently, one-dimensional (1D) semiconductor nanostructures have received much attention for their unique optical, electronic, mechanical and chemical properties and potential applications in optoelectronics, photonics, field emission, energy conversion, catalysis, and sensing.10-15 They provide more chances for researchers to understand the fundamental roles of dimensionality and quantum size effects. Meanwhile, 3D arrays of well-aligned 1D semiconductor nanostructure (nanowire, nanorod, or nanotube) with intensified anisotropy are highly desirable because of the requirement of many practical applications for nanodevice designs. Such arrays are potential building blocks for all kinds of electronic and optoelectronic nanodevices including laser diodes, field-effect transistors, and electron emitters.16-19 Inspired by these promising properties and applications, arrays of some semiconductors, mostly oxides such as ZnO, CuO, and SnO2, have been fabricated,20-22 and subsequent research results showed that these arrays possess excellent photoluminescence, field-emission, and sensing properties. A variety of ZnS-based 1D nanostructures, including nanowires, nanotubes, and nanoribbons, have already been prepared by various methods.23-28 However, there are few reports about the preparation of 1D ZnS array nanomaterial, which may be caused by the lack of practical synthetic routes or processing techniques. Up to now, only two literature accounts have * To whom correspondence should be addressed. Phone: 86-551-3606447. Fax: 86-551-3607402. E-mail:
[email protected]. † University of Science and Technology of China. ‡ The Chinese University of Hong Kong.
appeared on the synthesis of a ZnS array. Jiang et al.29 reported that crossed ZnS nanowire arrays have been grown on ZnS nanoribbons with a vapor-liquid-solid process during thermal evaporation, which involved high temperature, gold catalyst as an impurity in the final sample, and the presynthesized ZnS nanoribbon substrate. Lu et al.30 reported the synthesis of a ZnS nanobelt array by an ethylenediamine (EDA)-assisted solvothermal reaction to synthesized ZnS(EDA)0.5 precursor and subsequent heat treatment in vacuum for the thermal decomposition of ZnS(EDA)0.5. The above two approaches require either high energy consumption or a relative complicated procedure. It is understood that controlled growth or fabrication of nanostructrues with desired shapes plays a key role in both nanomaterials science and technology. However, it is usually difficult to control the nucleation and growth of nanomaterials for fabrication of well-controlled 3D nanostructures. To date, the controlled preparation of large-scale ZnS well-ordered 3D arrays of 1D nanostructure with desired shapes by a low cost and convenient method still remains a challenge. Here we report a facile one-step solution method for growing uniform wellaligned 1D ZnS nanostructure arrays at low temperatures. Moreover, single crystalline ZnS nanowires, nanotubes, or nanoribbons can be selectively synthesized by the controlling reaction temperature and sulfur concentration. Recently we investigated the structural degradation behavior of ZnS nanotubes.31 In this work, we focus mainly on the controlled growth of ZnS 1D nanostructure arrays without using any surfactant or template. By controlling the interfacial thermodynamics and kinetics of diffusion, nucleation, and growth, desired ZnS 1D nanostructures can be obtained. Experimental Section All reagents were analytical grade and used without further purification. In a typical procedure, an appropriate amount of sulfur was introduced into 15 mL of hydrazine hydrate (N2H4 · H2O, 85 wt %) with vigorous magnetic stirring until a transparent yellow solution appeared, giving a final sulfur concentration of 0.1 or 0.14 M. Then, the above solution was transferred into a 20-mL capacity Teflon-lined stainless steel autoclave. A piece of 1 cm × 2 cm zinc foil was treated by sonication in ethanol and acetone for 5 min respectively and dried with flowing nitrogen gas. Then the zinc foil was put into the autoclave and leaned against the inner wall of the autoclave. The autoclave was sealed and maintained at the desired temperature (100, 140, or 180
10.1021/cg800929h CCC: $40.75 2009 American Chemical Society Published on Web 03/23/2009
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°C) for 8 h before being cooled to room temperature. The zinc foil was taken out of the solution, rinsed with distilled water and ethanol, and air-dried for characterization The overall crystallinity of the product was examined by X-ray diffraction (XRD, Rigakau RU-300 with Cu ΚR radiation). The general morphology of the products was characterized using scanning electron microscopy (SEM, LEO 1450VP). Detailed microstructure analysis was carried out by using transmission electron microscopy (TEM, Philips CM120). The chemical composition analysis was obtained by energy dispersive X-ray spectrometry (EDX) using an EDX spectrometer attached to the same micoscope. Cathodoluminescence (CL) spectra were taken in a scanning electron microscope (Cambridge S360) using a MonoCL system (Oxford Instruments).
Results and Discussion After the reaction, a white layer was found on the zinc substrate. Some of the white layer was scraped off and collected for XRD characterization to obtain information regarding the crystal structure and phase composition. A representative XRD pattern of the as-prepared ZnS nanostructure is shown in Figure 1. All of the diffraction peaks can readily be indexed to the hexagonal wurtzite structure ZnS with the lattice constant a ) 3.818 Å and c ) 6.259 Å, which match well to the reported value, as listed in the JCPDS card, no. 36-1450. It is found that the relative intensity of the (002) peak is stronger than that reported in the JCPDS file, suggesting that the preferential orientation of the ZnS nanostructure is along the c-axis, which will be demonstrated by the TEM analysis in later sections. The product obtained from the reaction conducted at 100 °C with a sulfur concentration of 0.10 M was characterized by SEM. Figure 2a and b show SEM images of the as-prepared ZnS nanostructure on the Zn substrate. The low magnification SEM image (Figure 2a) gives a general morphology of the sample and represents that the foil is covered with a large-scale array of highly ordered nanowires, which is dense and uniform over the entire area. From the closer side view observation (Figure 2b), it can be seen clearly that these nanowires are all parallel to each other and oriented nearly vertically to the foil surface. The diameter of the nanowires is about 100 nm, and the length is in the range of 1-2 µm. Detailed microstructure information and chemical composition of an individual nanowire are obtained through TEM studies accompanied by selected area electron diffraction (SAED) and EDX. A typical TEM image of the nanowires is shown in Figure 2c, which confirms that the nanowires have a uniform diameter of 100 nm and the surface is smooth. The high resolution TEM (HRTEM) image of an individual nanowire (inset of Figure 2c) shows clear lattice spacing of 0.31 nm corresponding to the d spacing of the (0002) planes in wurtizite structure ZnS. The SAED of the nanowires is also displayed in the inset of Figure 2c, revealing wellcrystallized ZnS single crystals. Both the HRTEM and SAED results indicate that the nanowires grow preferentially along the [0001] direction and the lattice planes of the hexagonal phase stacked in that direction are closed-packed, which is consistent with the XRD results shown in Figure 1. The EDX spectrum (Figure 2d) recorded from the product shows intense peaks of Zn and S. The copper and carbon signals come from the supporting TEM grid. EDX quantitative analysis indicates the atomic ratio of Zn to S is 0.98:1, close to the stoichiometry of ZnS. The shape of the product was found to be sensitive to the reaction temperature. If the reaction is conducted at 140 °C with the sulfur concentration unchanged, instead of the nanowires array, a nanotubes array can be formed as the final product. Figure 3a shows a low magnification SEM image of the product with a top view, which reveals a wire-like nanostructure array
Figure 1. Representative XRD pattern of the as-prepared ZnS nanostructures.
on a large scale. The high magnification SEM image in Figure 3b give a tilt view of the product, indicating these “nanowires” are very uniform with an average diameter of ∼200 nm and length of up to 3-5 µm. Obviously, the “nanowires” are a little longer than those formed at 100 °C. Figure 3b illustrates also that these “nanowires” tend to bend and stick together to form bundles. This may be due to the increase in length and decrease in stiffness. Meanwhile, the capillary action among them may induce surface tension and force these long nanotubes to bend and bundle together during drying.32 The TEM image in Figure 3c shows the dark/light/dark contrast along the radial direction, giving evidence that these “nanowires” are actually nanotubes. The HRTEM image in Figure 3e displays also light/dark contrast near the wall of the nanotube along the radial direction, further confirming the tubular structure. The clear lattice spacing of 0.31 nm in the HRTEM, together with the SAED pattern in Figure 3d, indicates the single crystal nature of the ZnS nanotubes with a [0001] growth direction. The EDX spectrum in Figure 3f shows its chemical composition of only Zn and S in the as-prepared nanotubes array. When the reaction temperature and sulfur concentration are increased to 180 °C and 0.14 M, respectively, the final product became a nanoribbon array. An overview SEM image of the product in Figure 4a shows that a long wire-like nanostructure array covers the whole foil surface with honeycomb-like patterns originated from the bending of the one directional nanostructure. The magnified image in Figure 4b discloses their nanoribbons structure with a length of ∼5 µm. A TEM image in Figure 4c indicates that the ZnS nanotructures are actually nanoribbons with a typical width of about 200-400 nm. Some nanoribbons show a twisted section (see arrow in Figure 4c) where thinning is observed. This is a typical shape characteristic of a nanoribbon. The side of the twisted section reveals that the thickness of the nanoribbons is only about 20 nm. The nanoribbons are almost electron transparent in the TEM observation, resulting from their small thickness. The SAED pattern (inset of the Figure 4c) is recorded from the area marked by a rectangle on a single nanoribbon, indicating the single crystal nature of the ZnS nanoribbons. Out-of focus diffraction indicates that the nanoribbon grows along the [0001] direction. The EDX spectrum in Figure 4d shows its chemical composition of only Zn and S. In our experiment, hydrazine hydrate is critical to the formation of the ZnS. In terms of chemical reaction, hydrazine is necessary in helping the dissolution process of sulfur. Hydrazine also behaves as a strong reducing agent in solution
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Figure 2. SEM (a, b), TEM (c), SAED and HRTEM (inset of c) images and EDX spectrum (d) of the as-prepared ZnS nanowires array.
with pH > 7, resulting in S being readily reduced to active S2according to eqs 1 and 2.
N2 + 4H2O + 4e f N2H4 + 4OHS + 2e f S2-
E° ) -1.15 V (1)
E° ) -0.48 V
(2)
Meanwhile, the hydrothermal treatment releases Zn2+ from the zinc foil according to the follow reactions:
Zn + 2H2O f Zn(OH)2 + 2H+ + 2e
(3)
S + 2e f S2-
(4)
2H+ + Zn(OH)2 f Zn2+ + 2H2O
(5)
Here the released Zn2+ is coordinated by hydrazine to form zinc-hydrazine complex, [Zn(N2H4)]2+, which acts as the zinc precursor. During the reaction, the complexes [Zn(N2H4)]2+ will decompose thermally and continuously supply fresh zinc ions into solution to form ZnS. Subsequently, a heterogeneous nucleation for ZnS occurs on the zinc foil surface because the energy barrier is lower than that in the solution. Then, crystal growth along a preferential direction leads to one-dimensional ZnS nanostructures on the Zn substrate through a simply chemical liquid deposition approach. The above reactions (3-5) indicate clearly that the generated electron increases the potential of the redox reaction system. As a result, the reducing action of hydrazine, together with the generated electron, facilitates the formation of active S2- through an interfacial process. Therefore, a large quantity of excess sulfur ions exists in the solution. This is believed to be the driving force for the growth of the one-dimensional nanostructures array. Generally speaking, supersaturation in growing regions is favorable to anisotropic growth. Peng reported that if a reaction with two precursors was employed to synthesize semiconductor nanocrystals, an excess of the relative less reactive precursor often generated elongated nanoparticles with a higher aspect ratio.33 In our experiment, a large excess of sulfur provides the kinetic force
for the anisotropic growth along the c-axis of the ZnS wurtzite structure. The formed 1D nanostructures have the same surface polarization and surface repulsion effects relative to each other, resulting in the formation of arrays vertical to the foil surface. Our experiment shows that the shape of the ZnS onedimensional nanostructures is sensitive to the reaction temperature. During the process of chemical liquid deposition at the liquid-solid interface, the zinc precursors were supplied continuously from zinc foil in hydrazine solution. At a relatively lower temperature, the rate of zinc precursor diffusion is relatively faster than that of crystal growth at the liquid-solid interface. So, the concentration of zinc precursor is uniform throughout the growing region where the newly formed ZnS nucleate and grow at the entire liquid-solid junction, producing water-insoluble solid wires array. However, if the reaction proceeds at a higher temperature, the growth rate will increase with respect to the diffusion rate. The zinc precursor concentration will decrease dramatically near the top area of the 1D ZnS nanostructures, resulting in a diffusion-limited crystal growth. Finally, limitation of the growth of nanowires and the preferential growth of nanowalls come into being. Meanwhile, it is known that the ionic and polar wurtzite-type ZnS is constructed as thermodynamically stable hexagonal close packing of sulfur and zinc atoms in space group P63mc with zinc atoms in tetrahedral sites. The wurtzite ZnS features a basal polar sulfur (0001j) plane, a top tetrahedron corner-exposed polar zinc (0001) plane, and low-index planes (parallel to the c axis) consisting of nonolar {1000} planes. The “low-symmetry” nonpolar faces, with 3-fold coordinated atoms, are the most stable ones, the polar ones being metastable. In addition, an anisotropic growth of the crystal along the [0001] direction is preferable as the result of an inherent asymmetry along the c axis. On the basis of the above analysis, the formation of hollow tubes should reduce the top metastable areas and enlarge the lateral areas of the most stable low-index nonpolar surfaces. Therefore, it is structurally stable and energetically favorable to form ZnS hollow tubes during the diffusion-limited crystal growth process. Another kinetic effect can also explain the formation of ZnS hollow tubes. During the present case of the crystal growing process, the growth of
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Figure 3. SEM (a, b), TEM (c), SAED (d), HRTEM (e) images and EDX spectrum (f) of the as-prepared ZnS nanotubes array.
Figure 4. SEM (a, b), TEM (c), SAED (inset of c) images and EDX spectrum (d) of the as-prepared ZnS nanoribbons array.
the basal plane is perhaps so fast relative to the delivery of ions from the bottom of the tube that the growing edges of the rod consume all of the precursors before they can diffuse to the center of the rod. As a result, the hollow tubes are formed. If the reaction temperature is increased further to 180 °C and the sulfur concentration is increased to 0.14 M, a large amount of nanoribbons can be obtained. A higher temperature generally leads to the increase of the systemic free energy and the decrease of the critical size of a stable nucleus. In our experiment, the smaller critical nuclei size, together with the increased sulfur concentration, results in a higher density of nuclei and then a higher density array of one-dimensional nanostructures. Here the diffusion rate is increased with the increasing temperature
and sulfur concentration and bring to the nanowires growth in the solid-liquid interface initially. By prolonging the reaction time, the nanowires grow longer and cannot support themselves; moreover, the high density of the nanowires gives them more chance to contact each other. Once these nanowires stick together, the oriented attachment34 growth parallel to the c axis take place on the contact areas, leading to the elimination of interface between contacted nanowires. The oriented attachment growth reduces the surface energy significantly, hence providing the thermodynamic driving force for aggregated growth. With the oriented attachment growth continuing, the side crystal planes glue and fuse together with an Ostwald ripening process and finally form smooth nanoribbons.
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nanostructures. Compared to the CL feature of the nanoribbons and nanowires, the intensity of the green emission peak for the ZnS nanotubes is enhanced dramatically. Because of the absence of impurity in the present ZnS nanostructures, the observed green emission may originate from some self-activated centers, probably vacancy states or interstitial states.38 Point defects induced by the isolated Zn vacancies may also contribute to this green emission.39 It has been reported that defect emission in nanomaterials is surface-dependent because of the surface dominance of the defects.40,41 In addition, studies also showed that the defect luminescence grows at the expense of the band edge emission in small diameter nanowires and is expected to quench the band edge in the extreme cases.42 In our present case, the ZnS nanowires have small size and large surface area with respect to nanoribbons. This may induce the increased intensity of defect-related green emission and the weakening of the bandgap emission as a consequence. In the case of nanotubes, the appearance of inner surface increases the surfaceto-volume ratio significantly. As a result, the green emission is strengthened dramatically and the bandgap emission is quenched almost completely. Figure 5. TEM image taken from the sample obtained after 4 h of uncompleted reaction at 180 °C with sulfur concentration of 0.14 M. Inset: HRTEM image of the area marked with a rectangle between two attached ZnS nanowires
Figure 6. Room temperature cathodoluminescence (CL) spectra of the ZnS nanostructure arrays: (a) nanoribbons (b) nanowires (c) nanotubes.
A TEM image, shown in Figure 5, taken from the sample obtained after 4 h of uncompleted reaction at 180 °C gives evidence for the oriented attachment. Four ZnS wires attached with their side planes can be observed clearly. The inset HRTEM image of the fused area marked with a rectangle between two attached ZnS nanowires indicates clear lattice fringes, confirming the c axis growth direction. The lighter areas are the boundary between the wires, revealing uncompleted fusing. The optical properties of ZnS nanostructures have been reported to be sensitive to the synthetic conditions and crystal size and shape.35-37 In our present cases, the as-prepared ZnS nanostructures are all pure and obtained without any impurity phase. Figure 6 shows the room-temperature CL spectra of the as-prepared nanowires, nanotubes, and nanoribbons. A weak bandgap emission located at 340 nm for all as-prepared nanostructures can be observed in Figure 6 and its inset. The bandgap emission peak for the nanoribbons is somewhat stronger than that of the nanowires. The bandgap emission peak for the nanotubes is actually a broad bump, suggesting its very weak intensity. Besides the bandgap emission, a strong green emission centered at about 530 nm is observed for all the ZnS
Conclusions In summary, we have demonstrated a facile one-step wetchemical approach for controlled fabrication of large scale ZnS single crystalline 1D nanostructures array on Zn foil. The use of hydrazine hydrate as solvent is critical to the synthesis; chemical liquid deposition at the liquid-solid interface leads to the formation of the ZnS nanostructures array. Here the Zn foil acts as both the Zn source and deposition substrate. By simple modifying the reaction temperature and sulfur concentration, we can selectively grow highly oriented ZnS nanowires, nanotubes, or nanoribbons array. The growth mechanism of ZnS arrays with different shapes is also proposed. Compared to the ZnS nanowire and nanoribbon arrays, significant enhancement of luminescence for the nanotube array is observed. This controllable and convenient 3D array manufacturing method offers a promising technique for the design of novel ZnS-related electronic and optoelectronic nanodevices. Acknowledgment. This work was supported by the National Natural Science Foundation of China.
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